1. Field of the Invention
The invention relates to a hybrid vehicle, and more particularly to a hybrid vehicle provided with power source of two types, namely, an internal combustion engine (engine) and an electric motor (motor), as power sources for running a vehicle.
2. Description of Related Art
A demand has grown in recent years for on-board detection of air-fuel ratio imbalance abnormalities between cylinders in a vehicle having a multi-cylinder internal combustion engine installed therein (which is on-board diagnostics (OBD), and a movement to regulate such detection by laws has recently been observed. For example, where a fuel injection system of some cylinders fails, a large imbalance of air-fuel ratio occurs between the cylinders and exhaust emission of the vehicle is degraded. The abovementioned demand has been created to prevent such a vehicle from running.
The same demand also applies to hybrid vehicles having a multi-cylinder internal combustion engine installed therein. When an air-fuel ratio imbalance abnormality is detected, it is desirable that fuel injection amount be corrected to eliminate the state of abnormality in order to prevent subsequent degradation of emission.
A comprehensive research conducted by the inventors demonstrated that when an air-fuel ratio imbalance abnormality occurs in an internal combustion engine in a hybrid engine, this abnormality also affects the drive system of the electric motor.